Document managed prompt message placeholders

[api-clients-generation-pipeline]

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See DataDog/datadog-api-spec#6674 Test branch datadog-api-spec/test/alex/message-placeholders-api-spec

[dd-prapprover-prod-77c48c]

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• ✅ PR is eligible for auto-approval by rule datadog-api-spec-generated-clients - 2026-09-13T15:54:38Z
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[PROFeNoM]

The missing LLMObsPromptChatMessage / LLMObsPromptMessagePlaceholder imports are a Python generator traversal bug, now fixed in DataDog/openapi-transformer#425 with an application-boundary regression. The full schema generates all six missing imports with that patch. Please avoid a manual generated-file workaround; this PR should be regenerated after #425 merges.